In the growing concern with environmental conservation, the energy-saving activities are being accelerated also in the industrial fields of home-use refrigerators and air conditioners. In the sector of electric compressors for compressing the refrigerant gas, an increasing number of motors employ such a rotor provided with magnets built therein. A conventional method of magnetizing the magnets is disclosed in Japanese Patent Laid-Open Application No. 2002-300762. According to the method disclosed in the patent, the magnets built within a rotor are magnetized by an electric current delivered to the stator's coil.
In the following, the above conventional magnetizing method is described referring to FIG. 11 and FIG. 12. FIG. 11 shows a sealed vessel of conventional electric compressor (hereinafter referred to as compressor), sectioned vertically. FIG. 12 shows the conventional compressor, as viewed from the above, whose sealed vessel has been cut into two pieces. Sealed vessel 1 houses motor 5 and rolling-piston type compressing element 10 driven by motor 5. Motor 5 includes rotor 8 having built-in magnets 7 a pre-magnetization state and stator 9 which is fixed to inner surface of the sealed vessel. Compressing element 10's shaft 11 is coupled and fixed with rotor 8. Coil 13 of stator 9 is reinforced and fixed with an immersing varnish.
A method of magnetization in the above-configured compressor is described below.
Shaft 11 is fixed fastened at a certain specific position with no rotary motion allowed, and then a certain specific electric current is delivered to coil 13 of the stator. Magnets 7 at the pre-magnetization state are magnetized by the magnetic fields thus generated. This has been one of the generally-used methods for magnetizing the magnets built in a rotor.
However, in a case where a neodymium magnet or the like rare earth magnet is used for the magnet, it requires such magnetic fields several times as strong as compared with conventional examples where a ferrite magnet is used for the magnets.
Strength of magnetic fields is in proportion to the strength of electric current. So, it requires a big electric current to generate strong magnetic fields.
For generating a big current used for the magnetization, the electric energy is once stored in a capacitor, and then discharged instantaneously. At this moment of electric discharge, the coil wound around a stator is heated by the big current and at the same time ill-affected by the strong magnetic fields. As the result, the coil is liable to be deformed, or seriously damaged.
In the case of synchronous motors, among other motor types, where the rotor is provided with a secondary conductor disposed at the circumference of rotor's iron core portion, the magnetizing effects are retarded by the secondary conductor. As the result, it requires an electric energy of higher strength for the magnetization. This means that the conventional magnetizing method which makes use of an electric current delivered to a coil causes a substantial damage on the coil. Eventually, it will make it very difficult to use the magnetizing method in normal production. In order to prevent the damage the stator's coil in a varnish, in order to have the coil physically reinforced and fixed. However, immersing a coil in a varnish accompanies a substantial increase in the number of process steps, resulting in an increased cost. Meanwhile, a magnetized magnet can be inserted in a rotor if its magnetic force is moderate; but it may not be easy to insert a neodymium or the like rare-earth magnets, which have far stronger magnetic force, after they are magnetized. Such magnets readily stick onto the rotor's core iron and significantly deteriorate the efficiency of operation.
The present invention addresses the above problems, and aims to offer reliable compressors at lower cost. A magnetizing jig and a method for magnetization serving the above objective are also disclosed.